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signals: re-assign CLD_CONTINUED notification from the sender to reciever

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Based on discussion with Jiri and Roland.

In short: currently handle_stop_signal(SIGCONT, p) sends the notification to
p->parent, with this patch p itself notifies its parent when it becomes
running.

handle_stop_signal(SIGCONT) has to drop ->siglock temporary in order to notify
the parent with do_notify_parent_cldstop().  This leads to multiple problems:

	- as Jiri Kosina pointed out, the stopped task can resume without
	  actually seeing SIGCONT which may have a handler.

	- we race with another sig_kernel_stop() signal which may come in
	  that window.

	- we race with sig_fatal() signals which may set SIGNAL_GROUP_EXIT
	  in that window.

	- we can't avoid taking tasklist_lock() while sending SIGCONT.

With this patch handle_stop_signal() just sets the new SIGNAL_CLD_CONTINUED
flag in p->signal->flags and returns.  The notification is sent by the first
task which returns from finish_stop() (there should be at least one) or any
other signalled thread from get_signal_to_deliver().

This is a user-visible change.  Say, currently kill(SIGCONT, stopped_child)
can't return without seeing SIGCHLD, with this patch SIGCHLD can be delayed
unpredictably.  Another difference is that if the child is ptraced by another
process, CLD_CONTINUED may be delivered to ->real_parent after ptrace_detach()
while currently it always goes to the tracer which doesn't actually need this
notification.  Hopefully not a problem.

The patch asks for the futher obvious cleanups, I'll send them separately.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Oleg Nesterov 2008-04-30 00:52:44 -07:00 committed by Linus Torvalds
parent 3b5e9e53c6
commit e442055193
2 changed files with 25 additions and 10 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

6
include/linux/sched.h
View file

@ -554,6 +554,12 @@ struct signal_struct {
#define SIGNAL_STOP_DEQUEUED 0x00000002 /* stop signal dequeued */
#define SIGNAL_STOP_CONTINUED 0x00000004 /* SIGCONT since WCONTINUED reap */
#define SIGNAL_GROUP_EXIT 0x00000008 /* group exit in progress */
/*
* Pending notifications to parent.
*/
#define SIGNAL_CLD_STOPPED 0x00000010
#define SIGNAL_CLD_CONTINUED 0x00000020
#define SIGNAL_CLD_MASK (SIGNAL_CLD_STOPPED|SIGNAL_CLD_CONTINUED)
/* If true, all threads except ->group_exit_task have pending SIGKILL */
static inline int signal_group_exit(const struct signal_struct *sig)

29
kernel/signal.c
View file

@ -603,10 +603,8 @@ static void handle_stop_signal(int sig, struct task_struct *p)
* the SIGCHLD was pending on entry to this kill.
*/
p->signal->group_stop_count = 0;
p->signal->flags = SIGNAL_STOP_CONTINUED;
spin_unlock(&p->sighand->siglock);
do_notify_parent_cldstop(p, CLD_STOPPED);
spin_lock(&p->sighand->siglock);
p->signal->flags = SIGNAL_STOP_CONTINUED |
SIGNAL_CLD_STOPPED;
}
rm_from_queue(SIG_KERNEL_STOP_MASK, &p->signal->shared_pending);
t = p;
@ -643,25 +641,23 @@ static void handle_stop_signal(int sig, struct task_struct *p)
* We were in fact stopped, and are now continued.
* Notify the parent with CLD_CONTINUED.
*/
p->signal->flags = SIGNAL_STOP_CONTINUED;
p->signal->flags = SIGNAL_STOP_CONTINUED |
SIGNAL_CLD_CONTINUED;
p->signal->group_exit_code = 0;
spin_unlock(&p->sighand->siglock);
do_notify_parent_cldstop(p, CLD_CONTINUED);
spin_lock(&p->sighand->siglock);
} else {
/*
* We are not stopped, but there could be a stop
* signal in the middle of being processed after
* being removed from the queue. Clear that too.
*/
p->signal->flags = 0;
p->signal->flags &= ~SIGNAL_STOP_DEQUEUED;
}
} else if (sig == SIGKILL) {
/*
* Make sure that any pending stop signal already dequeued
* is undone by the wakeup for SIGKILL.
*/
p->signal->flags = 0;
p->signal->flags &= ~SIGNAL_STOP_DEQUEUED;
}
}
@ -1784,6 +1780,19 @@ relock:
try_to_freeze();
spin_lock_irq(&current->sighand->siglock);
if (unlikely(current->signal->flags & SIGNAL_CLD_MASK)) {
int why = (current->signal->flags & SIGNAL_STOP_CONTINUED)
? CLD_CONTINUED : CLD_STOPPED;
current->signal->flags &= ~SIGNAL_CLD_MASK;
spin_unlock_irq(&current->sighand->siglock);
read_lock(&tasklist_lock);
do_notify_parent_cldstop(current->group_leader, why);
read_unlock(&tasklist_lock);
goto relock;
}
for (;;) {
struct k_sigaction *ka;